Iron oxide pigments containing al

ABSTRACT

Aluminium-containing iron oxide pigments of the formula Fe2-xAlxO3 with x values from 0.01 to 0.25, characterized in that they possess an a* value of 30.5 to 32.5 CIELAB units and a b* value of 25.5 to 30.5 CIELAB units, measured in each case as full shade in the alkyd resin according to DIN EN ISO 787-25: 2007.

The invention relates to new Al-containing iron oxide pigments, to aprocess for producing them and to their use for the colouring of pastes,paints, plastics, paper and building materials.

PRIOR ART

High-grade red iron oxide pigments which represent the state of the artare customarily single-phase haematites having Fe₂O₃ contents of 96.5 wt% up to 99.5 wt %.

In the case of the direct red iron oxide pigments, up to 2.5 wt % ofwater is included, very largely bound within the haematite lattice,whereas the red pigments obtainable by calcining are free from water ofcrystallization and therefore have higher Fe₂O₃ contents.

Numerous processes corresponding to the prior art exist for producingthese red pigments, with recent years having seen various measuresallowing improvements to be achieved in terms of the pigment properties.

Processes possessing particular industrial significance are theCopperas, precipitation and Penniman processes and also the calcining ofiron oxide precursors based on goethite and magnetite.

Important fields for use of these red pigments are inks and paints(solventborne, aqueous and powder coatings), plastics, and also paperand laminates, with levels of pigmentation of up to around 35 wt %.

For measuring the colour properties of red iron oxide pigments, thereare long-established test methods, in which the colouredness of mediacoloured using red iron oxide pigments, such as test specimens ofplastic or paint systems, is measured.

Standard parameters established for measuring the colouredness of rediron oxide pigments include the parameters of what is called the CIELABcolour space. In this system, every perceptible colour within thisthree-dimensional colour space is defined by the colour locus with thecoordinates L* (lightness), a* (red-green value) and b* (yellow-bluevalue). The more positive an a* value, the greater the redness of thecolour, and the more positive a b* value, the greater the yellowness ofthe colour. The colour blue, in contrast, becomes stronger as the b*value becomes more negative. In addition to these parameters, thesaturation C_(ab)* (also called chroma, or chromaticity) is often alsostated. This value is a direct product of the values a* and b* andrepresents the square root of the sum of the squares of a* and b*. Thevalues a*, b*, L*, and C_(ab)* are dimensionless values which arecommonly identified as “CIELAB units”.

In the colorimetry of red iron oxide pigments, a test established forpaint systems is the test in a long-oil alkyd resin (in accordance withDIN EN ISO 11664-4:2011-07 and DIN EN ISO 787-25:2007). A possible alkydresin used was, formerly, Alkydal L 64 from Bayer. Since then, other,similar alkyd resins have been used, such as WorléeKyd P 151 from WorléeChemie GmbH.

The corresponding colorimetry in plastics takes place, for example, inpolyethylene (high-density polyethylene, HDPE) at a level ofpigmentation of 1 wt %.

Moreover, a frequent requirement for the colouring of plastics is thatthe colour properties should change very little on exposure to thetemperatures typically required when processing. An important criterionfor assessment in this respect is the change in the saturation C_(ab)*relative to the original value.

One red pigment already long established on the market is the CopperasRed® R 1599D from Huntsman. The b* value of this product, in particular,however, is still in need of improvement. The same is also true, forexample, of other red haematite pigments described in the prior art,such as those from WO 2016/038152 (see Table 1).

TABLE 1 Colour values in WorléeKyd P 151 (full shade, illuminantD65/10°) Red pigments as per Al prior art a* b* Cab* contents [%] R1599D (Huntsman) 31.4 24.5 39.9 — Example 1 31.1 24.7 39.6 — WO 2016/038152Example 3 31.3 23.0 38.8 1.0 DE 102004/024013 A1 Example 1 30.2 23.038.0 0.95 DE 2826941 A1 Example Fe:Al = 95:5 28.1 22.9 36.3 1.7 calcinedat opt. T = 700° C. from EP 1380542 EP 187331, Example 7 27.7 19.9 34.11.3 analogous production

Numerous attempts have already been undertaken in order to improvefurther these red pigments or to further improve their provision.

DE 3500470 (EP 187331), for example, attempted to use the precipitantMgO and a specific precipitation methodology in order to provide anAl-doped haematite having an improved hue (see Examples 7 and 8). Whileit did find as light increase in the b* value (see Comparative ExampleBversus A), as a result of using MgO as against NaOH in the Al-freehaematite, the absolute colour values found in DE'470 even for theA-doped haematites were still in need of improvement, especially the b*values. In any case, the presence of magnesium leads to the formation ofMg ferrites, which do not have good coloristic qualities. A comparisonof MgO and the variant addition described in EP'331 relative to theprocedure of the present invention can be found in Comparative ExampleIII of the present invention (for results see Table 1).

In EP 1380542 A1 as well, an attempt was made to adopt a pathway inorder to provide improved red pigments. Thus, starting from an ironnitrate/aluminium nitrate solution and using organic compounds, a gel isproduced which, after calcination, results in an Al-doped iron oxidewhich even at its optimum calcining temperature of 800° C. (see FIG. 4B)is still not very pure in colour, with an a* value of only 27.6 CIELABunits. In coloristic terms, moreover, this Al-doped pigment is also noimprovement on the pure haematite, which at its optimum calciningtemperature of 650° C. (see FIG. 4A) has a higher a* value of 29 CIELABunits all the same. Accordingly, even the pathway in EP'542 does notlead to the desired objective. In a reproduction of the pigment fromEP'542 with an Fe:Al ratio of 95:5, the values in Table 1 were obtainedat the optimum calcining temperature of 700° C. (see ComparativeExperiment II of the present invention).

DE 2826941 likewise describes Al-containing red pigments which areobtained by the nitrobenzene reduction process; again, however, redpigments are obtained whose coloristic qualities are in need ofimprovement (cf. Table 1 below). They are produced by precipitation ofan Al salt onto a magnetite precursor.

According to DE 102004/024013 A1, Al-containing red iron oxide pigmentsare produced by coating of finely divided goethite (α-FeOOH) precursorswith aluminium compounds, with subsequent calcination. The reworking,for example, of Example 3 in that specification yielded Al-containingred iron oxide pigments whose colour properties, however, are still inneed of improvement (Tab. 1).

Objective

It was an object of the present invention, therefore, to provide redpigments which expand the colour space relative to the red iron oxidepigments in the prior art. These new pigments are preferably to possessa higher saturation C_(ab)* as well and, in particular, an improved heatstability, in plastics, for example. It has been found that specificAl-containing iron oxide pigments achieve this object.

DESCRIPTION

The invention therefore relates to Al-containing iron oxide pigments ofthe formula Fe_(2-x)Al_(x)O₃ with x values from 0.01 to 0.25,characterized in that they possess an a* value of 30.5 to 32.5 CIELABunits and a b* value of 25.5 to 30.5 CIELAB units, measured in each casein the alkyd resin according to DIN EN ISO 787-25: 2007 as full shade.

Colorimetry on the Al-containing iron oxides of the invention here takesplace in a long-oil alkyd resin in accordance with DIN EN ISO787-25:2007, preferably with the illuminant D65/10°, for example inWorléeKyd P 151 from Worlée Chemie GmbH.

The Al-containing iron oxide of the invention is preferably present in ahaematite structure. In this case the aluminium is located preferably atthe octahedral lattice sites in substitution of Fe³⁺ ions.

Preferred Al-containing iron oxides have a saturation C_(ab)* of 39.8 to44.6 CIELAB units. C_(ab)* here represents the square root of the sum ofthe squares of a* and b*, measured in the varnish system above.

The pigments of the invention preferably possess a heat stabilitymeasured in HDPE polyethylene at 1% pigmentation, determined accordingto DIN EN 12877-2 by a change (ΔC_(ab)*) in the saturation (C_(ab)*) ofless than 3 CIELAB units, preferably less than 1.5 CIELAB units, ontemperature increase from 200 to 320° C.

In one preferred embodiment of the pigments of the invention, in theformula, the Al index x is a number from 0.01 to 0.10, more particularlyfrom 0.025 to 0.075.

Likewise preferred are pigments of the invention in which, in theformula, the Al index x is a number from 0.11 to 0.25, more particularlyfrom 0.12 to 0.15.

The pigments of the invention likewise preferably have a water contentof less than 0.8 wt %, preferably of less than 0.5 wt %.

In a further preferred embodiment, the pigments of the invention have achloride content of less than 0.1 wt %, preferably less than 0.01 wt %,based on the pigment.

The amount of manganese and chromium as well is preferably very small.The sum total of manganese and chromium is preferably less than 500 ppm,very preferably less than 100 ppm, based on the pigment.

For magnesium as well it is the case that its proportion is preferablyvery low. The amount of magnesium is preferably less than 500 ppm, verypreferably less than 100 ppm, based on the pigment.

The pigments of the invention preferably have a specific surface area bythe BET method of 6.5 to 12.5 m²/g.

The pigments of the invention may also be coated. In that case they mayhave one or more coatings selected from organic and/or inorganiccompounds.

Organic coating materials include, for example, polyhydric alcohols,polyethylene glycols, polypropylene glycols, their etherificationproducts with monohydric alcohols and esterification products withcarboxylic acids, and also silicone oils.

Suitable inorganic coating materials are preferably colourless oxides orhydroxides of Al, Si, Zr and Mg, especially Al₂O₃.

Where the pigments of the invention are coated, the coating materialsare employed preferably in an amount of 0.01 to 3 wt %, based on thepigment.

Process

The invention further relates to a process for producing the pigments ofthe invention, comprising at least the steps of a) precipitation, b)oxidation and c) calcination, characterized in that:

-   -   a1) an aqueous solution comprising ions of iron, of sulfate and        of aluminium, the molar ratio of iron ions to Al ions being        199:1 to 7:1, is reacted with an alkali metal hydroxide, such as        NaOH, LiGH or KOH, more particularly NaOH, as alkaline compound,        with the aqueous solution comprising ions of iron, of sulfate        and of aluminium being metered into the initial charge of the        alkaline compound, preferably in the form of its aqueous        solution, or    -   a2) an aqueous solution comprising ions of iron and of sulfate        is reacted with an alkali metal hydroxide, such as NaOH, LiGH or        KOH, more particularly NaOH, as alkaline compound, and with at        least one aluminium compound, preferably alkali metal aluminate,        more particularly sodium aluminate, with the molar ratio of iron        in the solution to aluminium in the aluminium compound being        199:1 to 7:1 and with the alkaline compound being introduced as        an initial charge with at least one aluminium compound,        preferably an alkali metal aluminate solution, in particular in        the form of an aqueous solution, and the aqueous solution        comprising ions of iron and of sulfate being metered in,    -   b) the aqueous suspension obtained after step a) is oxidized in        the presence of an oxidizing agent, and    -   c) the oxidation product obtained after b) is calcined at a        temperature of 500 to 1100° C. in an oxidizing atmosphere.

The Fe:Al ratio of 199:1 to 7:1 here corresponds, in the targetcomposition of Fe_(2-x)Al_(x)O₃, to an x value of 0.01 to 0.25.

Precipitation a1)

The aqueous solution comprising ions of iron, of sulfate and ofaluminium can be obtained by mixing corresponding sulfate-containingiron salt solutions with solutions containing aluminium ions, which inturn may be obtained individually from corresponding iron precursors andaluminium compounds, respectively.

For example iron(II) sulfates for such iron sulfate solutions can beobtained from steel-pickling plants or from TiO₂ production by thesulfate process, or by dissolving metallic iron, iron carbonates, ironhydroxides or iron oxides in sulfuric acid.

For producing the pigments of the invention it is preferred to use verypure iron raw materials in the form of iron(II) sulfate solutions havinga total iron content of 80 to 95 g/I and a sum content of manganese andchromium of less than 250 mg/I.

The solution used preferably also includes a magnesium content of lessthan 500, preferably less than 100 ppm, based on the solution.

Preferred for the precipitation according to step a) is an aqueoussolution comprising ions of iron, of sulfate and aluminium in which theiron ions are present in the form of a mixture of iron(II) and iron(III)ions, preferably with an Fe(III) fraction of 5 to 30 mol %, moreparticularly 10 to 20 mol % Fe(III), based on the total amount of ironin the solution.

Setting the correspondingly preferred Fe(III) fractions in therespective iron(II)/(III) sulfate mixture can be done either by addingcorresponding amounts of iron salts, preferably of iron(III) sulfate, orby partial oxidation of the iron salt solution, preferably the iron(II)sulfate solution, with—for example—atmospheric oxygen, preferably attemperatures of 80° C. or above, in particular at 80 to 100° C., or withH₂O₂ at temperatures preferably of 20 to 70° C.

Al components used in the aqueous solution comprising ions of iron, ofsulfate and of aluminium may be aluminium salts such as, for example,chlorides, sulfates or else nitrates, particular preference being givento Al(III) sulfates.

The aqueous solution comprising ions of iron, of sulfate and ofaluminium for step a1) preferably contains a molar ratio of iron in theform of Fe(II) and/or Fe(III) to Al ions of 79:1 to 26:1, preferably of17.2:1 to 7:1, more particularly of 15.7:1 to 12.3:1. An Fe:Al ratio of79:1 to 26:1 here corresponds, in the target compositionFe_(2-x)Al_(x)O₃, to an x value of 0.025 to 0.075; an Fe:Al ratio of17.2:1 to 7:1 here corresponds to an x value of 0.11 to 0.25; and anFe:Al ratio of 15.7:1 to 12.3:1 here corresponds to an x value of 0.12to 0.15.

The aqueous solutions comprising ions of iron, of sulfate and ofaluminium and used in accordance with the invention are providedpreferably by mixing of the Fe(III)- and/or Fe(II)-containing sulfatesolution and of corresponding Al-containing solutions.

The reaction in step a1) is preferably accomplished by heating thealkaline compound as precipitant in a suitable reaction vessel withstirrer, gasification container and electrical heating to the reactiontemperature.

The reaction temperature is preferably 20 to 100° C., more particularly80 to 100° C., more preferably 85 to 100° C.

The aqueous solution comprising ions of iron, of sulfate and ofaluminium is metered into the initial charge of the alkaline compound,preferably in the form of its aqueous solution. This addition ispreferably made at the reaction temperature.

The precipitation here takes place preferably at a pH of greater than10, more particularly at a pH of 10.5 to 14.

The addition is made preferably with stirring. If a particular ratio ofFe(II) and Fe(III) has already been set in the aqueous solutioncomprising the ions of iron, of sulfate and of aluminium, it ispreferred to allow the precipitation reaction to proceed under inertgas. Optionally, however, the Fe(II)/(III) ratio may also be set onlyduring the precipitation, by means of the above-described oxidation.

The amount of alkaline compound to be used for the precipitation is aproduct of the amounts of the iron ions and aluminium ions, preferencebeing given to a molar ratio of Fe_(total) to OH⁻ of 0.45 to 0.55 andalso of Al(III) to OH⁻ of 0.33, and also, optionally, of free acidpresent that is to be neutralized—sulfuric acid, for example.

Precipitation a2)

In a further preferred embodiment of the invention, the procedure andthe proportions of the iron ions to aluminium ions are fundamentally thesame as in the case of the precipitation a1), with the difference beingthat the aluminium compound is not present in the iron(II)/(III) sulfatemixture but is instead introduced as an initial charge together with thealkaline compound serving as precipitant.

The aluminium compound is preferably, for example, an aqueous Naaluminate solution which is mixed with the alkaline precipitant in orderthen to furnish the soluble Al ions.

Suitable alkaline compounds serving as precipitant are those specifiedunder a1).

The alkaline precipitant is preferably included as an initial chargemixed with an alkali metal aluminate solution, and the iron(II)/(III)sulfate mixture is metered into this initial charge.

Oxidation b)

The precipitation is followed by oxidation with an oxidizing agent. Theoxidizing agent used is preferably an oxygen-containing gas, such asair, for example. This oxidation takes place preferably in the aqueousmedium obtained after step a1) or a2), more particularly in thesuspension obtained as a result of the precipitation. The oxidizingagent, more particularly the oxygen-containing gas, is preferablyintroduced into the aqueous medium obtained after step a1) or a2).

The oxidation according to step b) here takes place in particular at atemperature of 20 to 100° C. more particularly at 80 to 100° C. verypreferably at 85 to 100° C.

The course of the oxidation and also the end of the oxidizing step canbe checked, for example, by an EMF measurement using a commercial redoxelectrode in the reaction vessel. The depletion of dissolved iron(II)ions in the reaction mixture is indicated by a jump in potential.

After oxidation has taken place, the pigment precursor, preferably themagnetite formed, is isolated by filtration and preferably washed, inparticular until the filtrate conductivity is below 2000 μS/cm,preferably below 800 μS/cm, more preferably below 200 μS/cm. This isfollowed preferably by drying of the filter cake, in particular at atemperature of 30 to 250, preferably of 30 to 120° C.

Calcining c)

The production of the Al-containing iron oxide pigments of the inventionwith the composition Fe_(2-x)Al_(x)O₃ is accomplished by calcination ofthe oxidation product obtained after step b), preferably in the form ofthe isolated, washed and dried filter cake, also referred to asAl-containing magnetite, at a temperature of 500 to 1100° C., preferablyof 600 to 975° C., preferably in the presence of an oxygen-containinggas, more particularly of air.

During the calcination according to step c) of the process of theinvention, it should be borne in mind that the level of optimumcalcining temperature is dependent on the Al content of the oxidationproduct obtained after step b). The optimum calcining temperature hereis the temperature at which the maximum a* value (red fraction) has beenobtained. This may be determined in a series of different calciningtemperatures.

In order to improve further the coloristic properties and also theprocessing properties in binders and plastics, the pigments of theinvention obtained after step c) may additionally be subjected togrinding and/or to coating.

In the case of an inorganic coating, it is preferred for coating tofollow step c). Preferred inorganic coating materials that are suitableare preferably colourless oxides or hydroxides of Al, Si, Zr and Mg,especially Al₂O₃.

It is likewise preferred for the Al-containing iron oxides of theinvention, with or without inorganic coating, to be subjectedadditionally to milling. Suitable milling methods are, for example, jetmilling, pendulum milling or else wet milling operations.

In the course of milling, it is possible with preference to add organiccoating materials, examples being polyhydric alcohols, polyethyleneglycols, polypropylene glycols, their etherification products withmonohydric alcohols and esterification products with carboxylic acids,and also silicone oils. These coating materials may likewise act asmilling assistants.

The preferred quantities of coating materials for metered addition maybe from 0.01 to 3 wt % in the case of inorganic coating materials andfrom 0.01 to 1 wt % in the case of organic coating materials. The sumtotal of organic and inorganic coating materials in this context is 0.01to 3 wt %.

Use

The invention further relates to the use of the pigments of theinvention for colouring pastes, paints, plastics, paper and buildingmaterials.

Measurement Methods Testing of Full-Shade Colorimetric Values

The full-shade colorimetric values were determined according to DIN ENISO 787-25:2007, using the test paste described below.

5 g of a thixotroped long-oil alkyd resin (WorléeKyd P 151) were appliedto the bottom part of a plate paint dispersion machine (TFAM) with aplate diameter of 240 mm, and the red iron oxide pigment in question wasprocessed with the test paste to form a coloured paste with a PVC(pigment volume concentration) of 10%.

The test paste contains 95 wt % of alkyd resin (Worléekyd P 151 fromWorlée-Chemie GmbH, DE) and 5 wt % of Luvotix HAT thixotropic agent(Lehmann & Voss & Co KG, DE). The Luvotix is incorporated by stirringinto the alkyd resin which has been preheated at 70 to 75° C., and themixed paste is heated at 95° C. until dissolution has taken place. Aftercooling, the paste is rolled free of bubbles on a triple-roll mill.

The red pigments were weighed out according to

$m_{P} = \frac{{PVC}*m_{b}*\rho_{p}}{\left( {100 - {PVC}} \right)*\rho_{b}}$

m_(P)=mass of red iron oxide pigmentPVC=pigment volume concentrationm_(b)=mass of binderρ_(p)=density of pigmentρ_(b)=density of binder

The completed paste was transferred to a paste plate and subjected tocolorimetry on a Datacolor 600 colorimeter with the measuring geometryof d/8° and the illuminant D65/10° with gloss (CIELAB colour spaceaccording to DIN 5033 Part 7).

Determination of the Heat Stability of Red Iron Oxide Pigments inPolyethylene (High-Density Polyethylene, HD-PE)

The heat stability in polyethylene (HD-PE) was tested by DIN EN 12 877-2according to method B in full shade.

HD-PE qrade: DOW KT 10000 UE (pellets)

Processing Equipment:

-   -   Schwabenthan Polytest 30 P single-screw extruder    -   Arburg 221 K-350-100 injection moulding machine

Colorimeter and Colorimetry

-   -   Datacolor 600        Measuring geometry d/8°        Illuminant D65/10° with gloss

Procedure

14 g of red iron oxide pigment were mulled with 1400 g of HD-PE pellets(1% pigmentation) in a polythene pouch in a PE drum for 20 minutes. Thebatch was subsequently extruded in the single-screw extruder at 180° C.and 60 rpm. These predispersed pellets (3 mm particle size) wereconverted in the above injection moulding machine into PE plaques withdimensions of 6*4 cm and a thickness of 3 mm. The start temperature was200° C. (likewise reference for the heat stability ΔC_(ab)*), and thetemperature was raised in 20° C. steps up to 320° C.

EXAMPLES Inventive Example 1

Conventional dissolution of metallic iron obtained electrolytically(commercial product of Allied Metals Corp.) and having an Mn and Crcontent of <1 ppm in each case, in sulfuric acid (96 wt %, ultra-pure,diluted with water; commercial product from Bernd Kraft) was used toprepare an iron(II) sulfate solution having an Fe²⁺ content of 92.15g/l, an Fe³⁺ content of 0.08 g/l, a free sulfuric acid content of 1.22wt % and a pH of 0.9 (solution 1).

Dissolution of metallic iron obtained electrolytically (commercialproduct of Allied Metals Corp.) and having an Mn and Cr content of <1ppm in each case, in ultra-pure sulfuric acid (commercial product fromBernd Kraft) was used to produce in the same way a second portion ofiron(II) sulfate solution. 12 mol of FeSO₄ in the form of this solutionwere reacted with 12 mol of hydrogen peroxide in the form of 946 ml of a35 wt % strength solution (commercial product from Merck) and with 5 molof H₂SO₄ in the form of the aforementioned 96 wt % strength acid, thetemperature rising from around 20 up to around 70°. This gave apredominantly iron(III)-containing sulfate solution with 64.6 g/l ofFe³⁺, 11.15 g/l of Fe²⁺ and 3.02 wt % of free sulfuric acid (solution2).

By mixing 12.45 l (15.4 kg) of solution 1, 3.12 l (3.82 kg) of solution2 and 396 ml (523 g) of Al₂(SO₄)₃ solution with an Al content of 4.3 wt% (commercial product of Feralco), an Fe(II)/Fe(III)/Al(III) sulfatemixture was produced which had a molar composition of 20.542 mol Fe(II),3.625 mol Fe(III) and 0.833 mol Al(III) and a total volume of 16.0 l.

A 30 l stirring vessel equipped with gasifier, heater, stirrer andliquid metering facilities was charged with 7.17 l of aqueous sodiumhydroxide solution (NaOH content 316 g/l) and this initial charge washeated to 90° C. with N₂ blanketing (80 l/h). The aforementionedFe(II)/Fe(III)/Al(III) sulfate mixture was metered into this alkalisolution at a uniform rate over the course of 45 minutes and at 90° C.with N₂ blanketing and stirring. After the end of the precipitationreaction, oxidation took place at a temperature of 90° C. within areaction time of 9.5 hours to form magnetite (oxidation product) by airintroduction (about 40 l/h).

A compilation of the batch quantities for the production of theoxidation product is given in Table 2.

The aqueous suspensions of the oxidation products were filtered in aknown way, washed to a filtrate conductivity <200 μS/cm andcharacterized as follows after drying of the filter cake at atemperature of 40° C.:

Specific surface area by BET method: 32.6 m²/gFe content: 67.9 wt %Al content: 1.0 wt %

The oxidation product thus isolated was calcined in a chamber kiln atthe optimum calcining temperature of 775° C. (accuracy ±5° C.) in aresidence time of 30 minutes under an oxidizing atmosphere (in thepresence of air). To determine the optimum calcining temperature, avariety of temperatures were trialled (see Table 3). The inventivepigment obtained was characterized—as indicated in Table 4—and testedcoloristically in WorléeKyd P 151 (full shade) (for colorimetric valuessee Table 5).

The same red pigment was processed in HD-PE and the heat stability wasascertained by measurement of the saturation C_(ab)* as a function ofthe processing temperature between 200 and 320° C. (see Table 6 and FIG.1).

Inventive Example 2

The procedure in this example was as for Inventive Example 1, with thesulfate mixture having a molar composition of 19.833 mol Fe(II), 3.5 molFe(III) and 1.667 mol A(III) and a total volume of 15.0 l. It wasobtained by mixing 10.84 l (13.5 kg) of an iron(II) sulfate solutionhaving an Fe²⁺ content of 99.21 g/l and Fe³⁺ content of 0.12 g/l and anH₂SO₄ content of 0.095 wt %; 3.03 l (3.705 kg) of solution 2 as inInventive Example 1; and 1.1 l (1.36 kg) of an Al₂(SO₄)₃ solution havingan Al content of 3.3 wt % (commercial product from Feralco). The initialcharge was 7.39 l of aqueous sodium hydroxide solution having an NaOHcontent of 316 g/l.

The oxidation time was around 10.5 hours at 85° C.

A compilation of the batch quantities for the production of theoxidation product is given in Table 2.

The characterization of this oxidation product after drying yielded thefollowing data:

Specific surface area by BET method: 35.8 m²/gFe content: 65.2 wt %Al content: 3.1 wt %

The oxidation product was calcined in a chamber kiln at the optimumcalcining temperature of 900° C. (accuracy ±5° C.) in a residence timeof 30 minutes under an oxidizing atmosphere. To determine the optimumcalcining temperature, a variety of temperatures were trialled (seeTable 3). The inventive pigment obtained was characterized—as indicatedin Table 4—and tested coloristically in WorléeKyd P 151 (full shade)(for colorimetric values see Table 5).

The same red pigment was processed in HD-PE and, as described inInventive Example 1, the heat stability was ascertained by measurementof the saturation C_(ab)* (see Table 6 and FIG. 1).

Comparative Example I

In this comparative example, an oxidation product without addition of Alwas produced in accordance with the procedure of Inventive Example 1.The sulfate mixture in this case had a molar composition of 21.25 molFe(II) and 3.75 mol Fe(III) with a total volume of 16.3 l (see Table 2).It was obtained by mixing 13.79 l (16.93 kg) of an iron(II) sulfatesolution having an Fe²⁺ content of 86.07 g/l, an Fe³⁺ content of 0.57g/l and an H₂SO₄ content of 0.86 wt %, and also 2.49 l (3.12 kg) of aniron(III) sulfate solution having an Fe³⁺ content of 84.12 g/l, an Fe²⁺content of 0.19 g/l and a free sulfuric acid content of 3.56 wt %. Theinitial charge was 7.53 l of aqueous sodium hydroxide solution with anNaOH content of 320 g/l.

After an oxidation time of around 7 hours at 85° C. and after work-up ofthe aqueous suspension, an oxidation product was obtained which had thefollowing data:

Specific surface area by BET method: 19.0 m²/gFe content: 70.4 wt %Al content: 0.01 wt %

This oxidation product was calcined at the optimum calcining temperatureof 700° C. (accuracy ±5° C.) with a residence time of 30 minutes in anoxidizing atmosphere in a chamber kiln. For the determination of theoptimum calcining temperature, a variety of temperatures were trialled(see Table 3).

The resulting non-inventive pigment without addition of Al wascharacterized—as indicated in Table 4—and tested coloristically inWorléeKyd P 151 (full shade) (for colorimetric values see Table 5).

The same red pigment was processed in HD-PE and, as described inInventive Example 1, the heat stability was determined by measurement ofthe saturation C_(ab)* (see Table 6 and FIG. 1).

The inventive Al-containing pigments from Inventive Examples 1 (0.81%Al) and 2 (2.2% Al) represent high-grade Al-containing red iron oxidepigments having specific surface areas by the BET method in the rangefrom 8.6 to 9.6 m²/g and they exhibit very high chemical purity,characterized by Mn and Cr contents of in total below 100 ppm, by Ccontents of below 250 ppm and by low H₂O contents of less than 0.01 wt %(see Table 5).

Relative to the four prior-art red haematite pigments, the colorimetricvalues of the inventive Al-containing pigments in WorléeKyd P 151 (fullshade) are significantly higher (see Table 5), thereby opening up newregions in the CIELAB colour space for red iron oxide pigments,specifically with:

Δa*=0.7 CIELAB unitΔb*=5.2 CIELAB unitsΔC*=4.0 CIELAB units

At the same time, in comparison to the prior art, the inventiveAl-containing red pigments are characterized by a significantly higherheat stability in HD-PE (as ΔC_(ab)* values of 200 versus 320° C., seeTable 6) with a significantly higher saturation C_(ab)*, specifically:

ΔC_(ab)* (inventive) up to −1.3 CIELAB units as against ΔC_(ab)* (priorart) of −1.7 to −2.3 CIELAB units

C_(ab)* (inventive) up to 43.4 CIELAB units as against C_(ab)* (priorart) of up to 39.4 CIELAB units

Comparative Example II

The Al-doped pigment described in EP-A 1380542 with the Fe:Al ratio of95:5 was reproduced in accordance with the data in the example there.While FIG. 4B of EP'542 does contain the values of a* and b* for thisexample at various temperatures, it does not include the precise methodby which this colour data was determined. The pigment produced accordingto the example of EP'542 was measured in the same way as for thedetermination of the colour data of the inventive examples above.

For determination of the optimum calcining temperature, a variety oftemperatures were trialled (see Table 3). In this case, as a furthertemperature relative to those of FIG. 4B from EP'542, 700° C. was testedas well, and emerged as being the optimum calcining temperature.

The noninventive pigment obtained was characterized—as indicated inTable 4—and tested coloristically in WorléeKyd P 151 (full shade) (forcolorimetric values see Table 5).

In the test system of the present invention as well, the a* and b*values for the Fe:Al=95:5 system produced according to EP'542 are welloutside the respective ranges of the invention.

Comparative Example III

Inventive Example 1 was repeated, but using MgO rather than NaOH asprecipitant, in half the molar quantity in accordance with the divalentnature of Mg, as employed for the production of Al-doped iron oxide inExample 7 of EP-A-187331. Moreover, the initial charge, rather than theNaOH, was the Fe(II)/Fe(III)/Al(III) sulfate mixture, and the MgOprecipitant was added to this initial charge likewise as described inEP'331. Accordingly, the differences of a different precipitant and adifferent sequence of addition were transposed from EP'331 to InventiveExample 1 of the invention.

The pigment produced was measured in analogy to the determination ofcolour data for the inventive examples above.

For determination of the optimum calcining temperature, a variety oftemperatures were trialled (see Table 3). A contrast with InventiveExample 2 is found in FIG. 2.

The noninventive pigment obtained was characterized—as indicated inTable 4—and tested coloristically in WorléeKyd P 151 (full shade) (forcolorimetric values see Table 5).

In the present test system of the present invention as well, the a* andb* values for the Example 7 produced in analogy to EP'331 are welloutside the respective ranges of the invention.

TABLE 2 Batch quantities for producing the oxidation products (step b)Examples FeSO₄/ Oxidation Batch quantities in mol NaOH products Me_(tot)Fe_(tot) Fe(II) Fe(III) Al(III) ratio Inventive 25 24.167 20.542 3.6250.833 0.541 Example 1 Inventive 25 23.333 19.833 3.5 1.667 0.484 Example2 Comparative 25 25 21.25 3.75 0 0.484 Example I

TABLE 3 Determination of the optimum calcining temperatures(colorimetric values in WorléeKyd P 151, full shade, illuminant D65/10°)Spec. Calcining surface area Colour values, temperature (BET) full shadePigment [° C.] [m²/g] a* b* Inventive Example 1 700 13.9 26.1 23.9 75011.8 27.5 24.0 775 8.6 31.9 25.9 800 7.5 31.8 25.1 825 5.7 30.7 22.4 8504.7 30.2 21.2 900 3.0 25.2 14.0 Inventive Example 2 700 14.2 31.4 28.9750 13.4 31.2 29.1 800 11.8 31.4 29.6 850 11.6 31.7 29.8 900 9.6 32.129.6 950 8.5 31.5 29.2 Comparative 600 7.9 29.1 21.0 Example I 650 7.129.8 21.1 700 6.2 30.1 20.7 750 5.7 30.0 20.0 800 4.9 29.4 18.8Comparative 300 26.7 24.3 19.8 Example II 500 20.4 25.8 20.5 650 17.127.8 22.6 700 14.9 28.1 22.9 800 10.3 27.7 23.2 1000 3.1 23.8 14.8Comparative 650 21.1 26.3 19.5 Example III 700 12.9 26.8 19.7 750 11.327.3 20.0 800 8.0 27.7 19.9 850 5.9 27.5 19.4 900 4.3 25.7 17.7 950 2.419.7 13.1

The optimum calcining temperature is highlighted by bold text.

TABLE 4 Characterization of the pigments of the invention aftercalcining of the oxidation products Optimum calcining temperatureSpecific for the red surface area Analytical values pigment (BET) Fetotal Al Mn Cr Cl Pigment [° C.] [m²/g] [wt %] [wt %] x* [ppm] [ppm][ppm] Inventive 775 8.6 69 0.81 0.047 <1 22 3 Example 1 Inventive 9009.6 66 2.2 0.129 2.1 13 8 Example 2 Comparative 700 6.2 70.4 <0.01<0.001 n.d.** n.d.** n.d.** Example I Comparative 700 14.9 n.d.** 1.7***0.1*** n.d.** n.d.** n.d.** Example II Comparative 800 8.0 58.9 1.3 0.0829** n.d.** n.d.** Example III *in relation to Fe_(2−x)Al_(x)O₃, **notdetermined ***as per data from EP'542

TABLE 5 Colorimetric values in WorléeKyd P 151 (full shade, illuminantD65/10°) Al contents a* b* C_(ab)* [wt %] x Pigments as per inventionInventive Example 1 31.9 25.9 41.1 0.81 0.047 Inventive Example 2 32.129.9 43.9 2.2 0.129 Comparative 30.0 20.7 36.5 <0.01 <0.001 Example IComparative 28.1 22.9 36.3 1.7*** 0.1*** Example II Comparative 27.719.9 34.1 1.3 0.8 Example Ill Red pigments as per prior art R1599 D 31.424.5 39.9 — Example 1 31.1 24.7 39.6 — WO 2016/038152 Example 3 31.3 2338.8 1.0 0.059 DE 102004/024013 A1 Example 1 30.2 23 38 0.95 0.056 DE2836941 A1 ***as per data from EP'542

TABLE 6 Data for FIG. 1 Example 1 Inventive Inventive DE ProcessingExample Example Comparative R 1599D 2836941 temperature 1 2 Example(Huntsman) A1 [° C.] C_(ab)* C_(ab)* C_(ab)* C_(ab)* C_(ab)* 200 38.943.2 34.3 38.8 39.4 220 39.2 43.3 34.7 38.9 39.3 240 39.3 43.4 35.1 38.639.1 260 39.3 43.1 34.8 38.4 38.6 280 39.2 42.9 34.4 38.1 37.9 300 3942.2 33.6 37.5 37.6 320 38.9 41.9 33.2 37.1 37.1 C_(ab)* ±0 −1.3 −1.1−1.7 −2.3 (for ΔT 200 to 320° C.)

1. Aluminium-containing iron oxide pigments of the formulaFe_(2-x)Al_(x)O₃ with x values from 0.01 to 0.25, characterized in thatthey possess an a* value of 30.5 to 32.5 CIELAB units and a b* value of25.5 to 30.5 CIELAB units, measured in each case as full shade in thealkyd resin according to DIN EN ISO 787-25:
 2007. 2. Pigments accordingto claim 1, characterized in that they possess a saturation Cab* of 39.8to 44.6 CIELAB units, in each case as full shade, measured in the alkydresin according to DIN EN ISO 787-25:
 2007. 3. Pigments according claim1, characterized in that they have a specific surface area, by the BETmethod, of between 6.5 and 12.5 m²/g.
 4. Pigments according to claim 1,characterized in that the aluminium-containing iron oxide pigment ispresent in a haematite structure.
 5. Pigments according to claim 1,characterized in that the sum content of manganese and chromium is lessthan 500 ppm, based on the pigment.
 6. Pigments according to claim 1,characterized in that the amount of magnesium is less than 500 ppm,based on the pigment.
 7. Pigments according to claim 1, characterized inthat the pigments possess a water content of less than 0.8 wt %. 8.Pigments according to claim 1, characterized in that the pigments havean organic and/or inorganic coating, more particularly in an amount of0.01 to 3 wt %, based on the pigment.
 9. Pigments according to claim 1,characterized in that they have a heat stability measured in HDPEpolyethylene at 1% pigmentation, determined according to DIN EN 12877-2by a change (ΔC_(ab)*) in the saturation (C_(ab)*) of less than 3 CIELABunits, on temperature increase from 200 to 320° C.
 10. Pigmentsaccording to claim 1, characterized in that in the formula the Al indexx is from 0.01 to 0.10, more particularly from 0.025 to 0.075. 11.Pigments according to claim 1, characterized in that in the formula theAl index x is from 0.11 to 0.25, more particularly from 0.12 to 0.15.12. Process for producing the aluminium-containing haematite pigmentsaccording to claim 1, comprising at least the steps of a) precipitation,b) oxidation and c) calcination, characterized in that: a1) an aqueoussolution comprising ions of iron, of sulfate and of aluminium, the molarratio of iron ions to Al ions being 199:1 to 7:1, is reacted with analkali metal hydroxide, more particularly NaOH, as alkaline compound,with the aqueous solution comprising ions of iron, of sulfate and ofaluminium being metered into the initial charge of the alkalinecompound, or a2) an aqueous solution comprising ions of iron and ofsulfate is reacted with an alkali metal hydroxide, more particularlyNaOH, as alkaline compound, and with at least one aluminium compound,more particularly sodium aluminate, with the molar ratio of iron in thesolution to aluminium in the aluminium compound being 199:1 to 7:1 andwith the alkaline compound being introduced as an initial charge with atleast one aluminium compound, and the aqueous solution comprising ionsof iron and of sulfate being metered in, b) the aqueous suspensionobtained after step a1) or a2) is oxidized in the presence of anoxidizing agent, and c) the oxidation product obtained after b) iscalcined at a temperature of 500 to 1100° C. in an oxidizing atmosphere.13. Process according to claim 12, characterized in that the aqueoussolution comprising ions of iron, of sulfate and of aluminium in stepa1) has an Fe(III) fraction of 5 to 30 mol %, more particularly 10 to 20mol % Fe(III), based on the total amount of iron in the solution. 14.Process for the colouring of pastes, paints, plastics, paper andbuilding materials, wherein the pigments according to claim 1 areutilized.
 15. Pigments according to claim 1, characterized in that theypossess a saturation Cab* of 39.8 to 44.6 CIELAB units, in each case asfull shade, with illuminant D65/10°, measured in the alkyd resinaccording to DIN EN ISO 787-25:2007.
 16. Pigments according to claim 1,characterized in that the sum content of manganese and chromium is lessthan 100 ppm, based on the pigment.
 17. Pigments according to claim 1,characterized in that the amount of magnesium is less than 100 ppm,based on the pigment.
 18. Pigments according to claim 1, characterizedin that the pigments possess a water content of less than 0.5 wt %. 19.Pigments according to claim 1, characterized in that they have a heatstability measured in HDPE polyethylene at 1% pigmentation, determinedaccording to DIN EN 12877-2 by a change (ΔC_(ab)*) in the saturation(C_(ab)*) of less than 1.5 CIELAB units, on temperature increase from200 to 320° C.
 20. Process according to claim 12 for producing thealuminium-containing haematite pigments according to claim 1, comprisingat least the steps of a) precipitation, b) oxidation and c) calcination,characterized in that: a1) an aqueous solution comprising ions of iron,of sulfate and of aluminium, the molar ratio of iron ions to Al ionsbeing 199:1 to 7:1, is reacted with an alkali metal hydroxide, moreparticularly NaOH, as alkaline compound, with the aqueous solutioncomprising ions of iron, of sulfate and of aluminium being metered intothe initial charge of the alkaline compound in the form of its aqueoussolution, or a2) an aqueous solution comprising ions of iron and ofsulfate is reacted with an alkali metal hydroxide, more particularlyNaOH, as alkaline compound, and with alkali metal aluminate, moreparticularly sodium aluminate, with the molar ratio of iron in thesolution to aluminium in the aluminium compound being 199:1 to 7:1 andwith the alkaline compound being introduced as an initial charge with analkali metal aluminate solution, and the aqueous solution comprisingions of iron and of sulfate being metered in, d) the aqueous suspensionobtained after step a1) or a2) is oxidized in the presence of anoxidizing agent, and e) the oxidation product obtained after b) iscalcined at a temperature of 500 to 1100° C. in an oxidizing atmosphere.